1. Field of the Invention
The present invention relates to a method and apparatus of forming corrosion protection coatings on prestressing strands to be used as tensioning members in a prestressed concrete structure, and particularly to a method and apparatus of forming individually protected strands by synthetic resin coatings.
2. Description of Related Art
To provide prestress in concrete by means of a pre-tensioning method or post-tensioning method, prestressing strands are used as tensioning members. At present, as such a prestressing strand, it is customary to use one having no corrosion protection coatings thereon.
Achievement of prestress in the concrete is mainly attributable to the bond between the surrounding concrete and the prestressing strand surfaces, specifically the helical dents of the strand surfaces formed by twisting of the side wires. Therefore, the forming of corrosion protection coatings on prestressing strand will reduce appreciably the width and depth of every helical dent of the strand, accordingly reducing the bond of the strand surfaces to the surrounding concrete.
To prevent reduction of the bond of the prestressing strand surfaces to the surrounding concrete, Japanese Patent 59-130960(A) (corresponding to U.S. patent application Ser. No. 437,274) proposed "anti-corrosion strand for use in prestressed concrete structure". It teaches a strand which has thick synthetic resin coatings on the strand surfaces and sand particles being blown against the coatings to be partly buried and exposed.
Conventional prestressing strands, however, have anti-corrosion coatings only on their outer surfaces, and no coatings are applied to the spaces between the core steel wire and the surrounding steel wires. If there should be pinholes in the anti-corrosion coating of the prestressing strand, damp air or water would invade inside of the coating through its pinholes, thereby corrosion could occur within in the core and surrounding wires.
As for the conventional prestressing strand having a sand-buried coating thereon, disadvantageously extra work is required for attaching sand particles on the strand, and if such rough-surfaced strands are gripped and pulled with hands, there is a fear of hurting the hands.